There is in wide commercial use today a large number of catalytic agents, useful in polymerization reactions, which possess the disadvantageous trait of being readily hydrolyzable. Because of the water sensitive nature of these catalysts, extreme care must be taken to ensure that the reaction systems in which they are employed are kept anhydrous.
For example, tetrabutyl titanate is well known as a catalyst for esterinterchange polymerizations. U.S. Pat. No. 3,763,109 discloses that segmented thermoplastic copolyesters containing recurring long chain ester units derived from dicarboxylic acids and long chain glycols and short chain ester units derived from dicarboxylic acids and low molecular weight diols may be produced via an esterinterchange reaction wherein tetrabutyl titanate is used as a catalyst. Similarly, U.S. Pat. No. 4,262,114 discloses that certain polyester-polyether segmented copolymers which find use as elastomers may also be produced by way of a polymerization reaction utilizing this catalyst.
While tetrabutyl titanate is an excellent polyesterification catalyst, it must be more or less completely protected from exposure to moisture in order to remain effective due to its extremely hydrolytic nature. It has been reported by Boyd (Boyd, T., Journal of Polymer Science, Vol. Vii, No. 6, pp. 591-602, 1951) that when tetrabutyl titanate is exposed to atmospheric moisture at room temperature, it hydrolyzes first to a dimer which subsequently may further react to form a linear polymer. When a substantial amount of water is present (two moles of water per mole of tetrabutyl titanate), the linear polyorthotitanate esters further hydrolyze to TiO.sub.2. It is suspected that the dimeric and polymeric hydrolysis products exhibit greatly reduced catalytic activity, if any at all, and TiO.sub.2, of course, is totally ineffective as a catalyst.
We have found that if tetrabutyl titanate which has been partially hydrolyzed is used as catalyst, both the rate of reaction and the degree of polymerization is decreased and, in addition, that the resulting polymer may have an undesirable color.
Although water is not evolved in esterinterchange polyesterifications, it is possible for water to be introduced as a contaminant of the reacting species. Accordingly, when tetrabutyl titanate is used as a catalyst, it is imperative that the reactants be substantially anhydrous. Where anhydrous starting materials cannot be economically obtained, they must first be dried. We have found, in particular, that moisture levels in excess of 0.2 wt. % of total starting materials will adversely effect reactions when tetrabutyl titanate is employed as a catalyst.
A great many other catalysts, particularly metal containing catalysts, are similarly hydrolyzable in nature. This class of catalysts includes, especially, metal oxides, metal alkoxides, organometallic compounds (including Grignard reagents), organic metal salts and inorganic metal salts. As a general rule of thumb, if a catalyst contains a metal which forms a water insoluble oxide or hydroxide, then the catalyst will be readily hydrolyzable. Organometallic compounds, since they contain a metal to carbon bond, are uniformly sensitive to moisture.
Representative members of this class of catalysts are:
GeO.sub.2 PA1 Sb.sub.2 O.sub.3 PA1 M(Al(OR).sub.4) PA1 M(HZr(OR).sub.6),M'HZr(OR).sub.6).sub.2 PA1 MH(Ti(OR).sub.6),M.sub.2 (Ti(OR).sub.6) PA1 M'(HTi(OR).sub.6).sub.2,M'(Ti(OR).sub.6) PA1 (RR'R"R"'N).sub.2 (Ti(OR).sub.6) PA1 (RR'R"R"'N)H(Ti(OR).sub.6) PA1 Ti(OR).sub.4,PbR.sub.4 ; and PA1 RMgX
wherein M represents an alkali metal, M' represents an alkaline earth metal selected from the group consisting of magnesium, calcium and strontium, R represents an alkyl group containing from 1 to 6 carbon atoms, R',R" and R"' each represents a member of the group consisting of R and an aryl group of the benzene series containing from 6 to 9 carbon atoms and X represents a halogen atom.
Representative polymerization reactions utilizing catalysts of the above described class are described in the following U.S. Pat. Nos.: 4,262,114; 3,763,109; 3,377,320; 3,008,933; 2,808,390; 2,805,213; 2,794,795; and 2,744,088 through 2,744,097.
As stated before with respect to the catalysts tetrabutyl titanate, in order to protect water sensitive catalysts, would not be useful for protecting water sensitive catalysts during the course of a reaction.
Surprisingly, we have found that aluminum sulfate is able to bind water extremely tightly and effectively, even at these typical polymerization reaction temperatures. We have also found that this dessicant does not in any way interfere with the course of polymerization reactions or with catalytic activity and that it may, accordingly, be allowed to remain in the reaction starting materials throughout the course of a reaction. Further, we have found that if powdered aluminum sulfate is used, having a particle size which is preferably less than about 5 microns, the dessicant may be left in the reaction product with few, if any, adverse effects. Thus, for example, fine denier synthetic fibers may be melt-spun from polymer containing such finely divided dessicant without clogging of the extrusion spinneret. In order to effectively remove water from reaction starting materials, we have found it preferable to use at least about 500 ppm and up to about 1.5 wt. % of aluminum sulfate, based upon the weight of starting materials.